Wireless local-area networks (WLAN) have become increasingly popular for communicating data between electronic devices. WLANs are frequently operated in infrastructure mode, where a wireless access point (AP) relays data between the electronic devices, which are referred to as wireless clients or stations (STA). Each communication between two STAs in infrastructure mode requires two sequential transmissions: from the sending STA to the AP, and from the AP to the receiving STA. In many cases, a direct link between the two STAs would be preferable.
Tunneled direct link setup (TDLS), which is described in IEEE 802.11e and 802.11z, provides this capability. FIG. 1 shows a conventional implementation of TDLS in an IEEE 801.11 WLAN 100. WLAN 100 includes a wireless AP 102 passing traffic between two STAs 104A,B in infrastructure mode over links 106A,B respectively. TDLS allows STAs 104A,B to establish a direct link 108.
FIG. 2 is a signaling diagram for conventional TDLS discovery in WLAN 100 of FIG. 1. In FIG. 2, time progresses from top to bottom. AP 102 transmits beacons B at a predetermined beacon interval. A common beacon interval of 100 ms is illustrated in FIG. 2. Every Nth beacon B includes a delivery traffic indication message (DTIM) D. In FIG. 2, N=2 for a DTIM interval of 200 ms. In FIG. 2, beacons containing a DTIM are shown as BD. STAs in sleep mode awaken to receive each DTIM. The DTIM informs each STA of any packets buffered at the AP and awaiting delivery to the STA. If the DTIM informs a STA that packets are waiting, the STA remains awake to receive the packets from the AP. If the DTIM informs a STA that no packets are waiting, the STA can return to sleep mode until the next DTIM.
TDLS begins when a STA (STA 104A in FIG. 2) has a TDLS trigger event T. The TDLS trigger event can be a user action, logic which determines that it is useful to have a direct link, or the like. In conventional implementations, STA 104A sends a TDLS discovery request frame 204 to AP 102 immediately after trigger event T, as shown in FIG. 1. Because STA 104B may be in sleep mode with respect to AP 102, AP 102 does not transmit the TDB discovery request frame 204 until alter the next DTIM, when STA 104B will be awake. If STA 104B does not support TDLS, it sends no response. If STA 104B supports TDLS, it sends a TDLS discovery response frame 206 directly to STA 104A (not via AP 102). If STA 104A receives a TDLS discovery response frame 206 from STA 104B, the STAs 104 can establish a direct link 108.
In either case, STA 104A must remain awake to either receive any TDLS discovery response frames 204 or determine that no TDLS discovery response frames 206 were sent. As shown in FIG. 2, the minimum awake interval for STA 104A could be longer than a DTIM interval. In FIG. 2 the DTIM interval is 200 ms, but in other implementations the DTIM interval could be much longer. This results in a large power expenditure by STA 104A. And because many STAs are battery-powered, such power expenditures are especially undesirable.